Various attempts have been made to fit portable sanding or like machines with vacuum exhaust shrouds for purposes of collecting dust generated during a sanding or other dust generating operation. As by way of example, U.S. Pat. Nos. 1,800,341 and 4,145,848 propose the use of shrouds having means arranged to engage with or seal against a rear surface of a rotary sanding member or pad formed with vacuum exhaust apertures affording flow communication with the working or sanding surface of such member. Further, it has been proposed for instance in U.S. Pat. Nos. 2,000,930 and 3,785,092 to provide shrouds having means to engage with the surface of a workpiece in order to provide a constricted dust laden air flow path disposed peripherally of the sanding or like member, as well as through exhaust apertures extending between rear and working surfaces of such member where provided therein.
Vacuum exhaust shrouds designed to engage with a sanding member or the surface of a workpiece being sanded by such member have the advantage that the pressure head required to be established by a vacuum source need be relatively small, so as to permit utilization of a relatively inefficient aspirator or an exhaust fan system formed as an integral part of the machine. However, a decided disadvantage of prior shroud constructions of this general type is that when a vacuum head is established, which is sufficient for dust collection purposes, there is a marked tendency for a shroud, particularly when formed of air impermeable resiliently deformable material, to seize or be drawn tightly against the surface with which it operatively engages, so as to retard and in some cases even arrest movement of the sanding member or movement of the machine over the surface of the workpiece.
Still further, as evidenced by U.S. Pat. Nos. 2,156,824 and 2,929,177, various shroud arrangements have been proposed, wherein the lower end portion of the shroud, which defines its inlet opening, is intended to be physically spaced from both the sanding member and the workpiece so as to provide for the unobstructed or free flow of dust laden air peripherally of a sanding member. These shroud constructions suffer from the disadvantage of requiring a large volume of air flow through the shroud, such as to preclude their use with sanding machines relying solely on a built-in aspirator or the like to create a vacuum condition.